BETWEEN THE STARS

Abstract

In discussing what is or may be found in interstellar space it is of interest first to try to gain some idea of the volume of interstellar space as compared to that occupied by the stars. A photograph of a rich star field or a star cluster is very apt to give the impression that space is about as well filled with stars as the air is with raindrops in an average shower. A very little consideration shows that this impression is quite erroneous. If similar objects, such as spheres, are scattered more or less uniformly through a given volume, the ratio of the occupied to the unoccupied space will be approximately equal to the cube of the average angular diameter of one object as viewed from one of its immediate neighbors. For the case of the stars in our galaxy this angular diameter is less than O'.'l or S X 10~7, the cube of which is of the order of 10r19. In other words, the unoccupied space in our galaxy is at least 1019 times as great as the combined volume of all the stars. Raindrops one-eighth of an inch in diameter would have to be on the average four miles apart in order to furnish a reasonable comparison. In order to judge how well the universe is filled with galaxies, we find from recent work on these objects that their average distance apart is of the order of 106 light-years, while their diameters appear to be roughly of the order of 104 lightyears. This gives 102 as the average angular diameter of a galaxy as seen from its neighbor or 10~6 as the fraction of space occupied by galaxies. Since the external galaxies presumably consist of stars distributed more or less as are the stars in our